This invention relates to the manufacture of thermoplastic films. More particularly, this invention relates to thermoplastic films having minute perforations on a regularly arranged pattern over substantially the entire surface of the film which have increased liquid flow rates through their perforations and are useful in making sanitary and apparel structures or assemblies, and particularly for making infant diapers.
Many thermoplastic films of different strengths, characteristics, colors, and dimensions are produced for a multitude of products. Perforated films have a multiplicity of regularly spaced apertures which allow permeation of liquid and air or other fluids. Such films can be used as a component of disposable composition garments for sanitary apparel purposes, such as napkins, diapers, or for hospital pads, bed or sleeping bag linings, and the like. In such composite structures, an exterior layer of film having the desired properties is provided which would be adjacent to the skin in a composite garment, and the garment would also include a filler layer or layers of absorbent fibrous material.
A particular class of perforated film is described by Thompson, U.S. Pat. No. 3,929,135, issued Dec. 30, 1975. Thompson teaches an absorptive structure with a top layer of perforated film characterized by having a series of regular spaced small apertures in the form of tapered capillaries of certain dimensions ranges. In the finished article, these are directed inwardly to be in intimate contact with an absorbent fibrous material layer. The smooth side of the perforated film is thus in use in contact with the skin. Film as described by Thompson, in garment structure as outlined, maintains a dry and comfortable condition, even after transmission of fluids to the absorbent layer by the combined effects of the absorption and the resistance to back flow as a result of the relative length and surface properties of the tapered capillaries.
One method for making film with tapered capillaries on one side thereof is shown in U.S. Pat. No. 3,054,148, issued Sept. 18, 1962, to Zimmerli. In this patent heated film is supported by a perforated screen and a vacuum applied to the underside of the perforated screen. Holes are pulled in the film in the direction of the vacuum beneath the screen thereby forming tapered capillaries in the film.
An alternative technique for forming tapered capillaries is disclosed in U.S. Pat. No. 3,394,211, issued July 23, 1968, to McDuff. In the McDuff process the perforation technique comprises applying a blast of heated air to create a pressure differential across a perforated forming surface, conditions being appropriate to cause a rupture at a desired point.
Furthermore, film may be perforated by a variety of other means including mechanical perforation by pins or the like, and by electrical means such as corona discharge. U.S. Pat. No. 3,471,597 discloses a method for perforating a film by corona discharge and U.S. Pat. No. 3,880,966 discloses a method of increasing the permeability of a film with corona discharge. However, some, if not all, of such films do not have tapered capillaries.
An object of the present invention is to increase the flow of liquids through a perforated film having tapered capillaries.